Many types of dry, finely-divided (i.e., powdered or powder-like) materials, e.g., flour, rice hulls, pharmaceuticals, plastic, colorant, cement among many, many others, are shipped to a point of usage using large, soft-sided, reusable bags. Such bags, commonly cubic, have a top fill opening and a bottom spout and after being emptied, the soft bags may be collapsed to but a fraction of their filled volume for return to a filling site.
At the point of usage, such bags are mounted one-at-a-time on a machine of the type generally known as a bag unloader or bag frame. There, the contents of the bag are emptied through the spout for, e.g., repackaging into retail containers, compacting into tablet form, mixing with other constituents or for other, further processing. A representative list of companies selling (or which have sold) bag unloading equipment includes National Bulk Equipment, Inc. (NBE), Holland, Mich.; Vac-U-Max, Belleville, N.J.; and Beta Raven, Inc. of Earth City, Mo. And Schenck AccuRate, Inc., Whitewater, Wis., is a leading manufacturer of such equipment.
A characteristic of bag-confined, finely-divided or powdered materials is that when the bag is full or nearly so and the spout is opened for material discharge, the material tends to "bridge," i.e., to repose in an arch-like formation. While such formation is structurally quite weak, it is sufficient to impede material flow. Similarly, such materials may "rat hole," i.e., may flow from the spout in such a way that a downwardly extending hole forms. This, too, impedes material flow.
This characteristic of dry powder materials has long been recognized and steps have been taken to prevent such bridging and rat holing. NBE uses a pair of double-faced paddles moved by a common cylinder to agitate the powder in a bag. NBE literature indicates this arrangement is to promote powder flow if bagged material becomes packed during transportation or storage. Another arrangement available from NBE involves arm-mounted hammers which periodically strike the sides of the bag.
Another aspect of prior art involves what is often referred to as a bag tensioning or bag lifting mechanism. One known type of such mechanism supports the bag using what are known as gas cylinders or, more typically, gas springs. (As an illustration, a gas spring is often used to automatically raise the rear hatch of a hatchback auto when unlatched.) The known type of mechanism uses four such gas springs.
While these prior art arrangements have been generally satisfactory for their intended purposes, they are not without disadvantages. As a general observation, such prior art arrangements seem not to optimally address the need to help assure smooth powder flow during all phases of bag emptying, i.e., at initial discharge, when the bag is about half empty and when the bag is nearly empty.
More specifically, known prior art arrangements do not optimally prevent close-to-the-spout material bridging at the onset of material flow from a fully-filled bag. Nor do such arrangements optimally prevent bridging and rat-holing well upward in the bag as emptying progresses. And such arrangements have not fully recognized how to deal with the tendency of material to lodge in the bag lower portions when the bag is nearly, but not entirely, empty.
A difficulty with the aforedescribed bag tensioning or bag lifting mechanism is that the gas springs sometimes tend to bind. And there is no opportunity to control operation by manipulating gas pressure; the gas spring is permanently sealed and, for a particular rod position, such pressure is fixed.
And aside from the matters of assuring smooth powder flow during all phases of bag emptying, prior art arrangements seem not to have appreciated how smooth, substantially bridge-free bag unloading can be improved by process control which changes the operation of a bag unloading machine as a function of the degree to which the bag is emptied.
An improved bag unloading machine and method which address these deficiencies perceived in prior art arrangements would be an important advance in this field of technology.